Polyamides refer to various natural (polypeptides) and synthetic materials containing free amino groups. Examples of polyamides include nylons, wool, and silk. These materials have many different and diverse uses, especially in the field of textiles. For instance, natural and synthetic polyamide fibers are commonly used to produce fabrics and carpets.
During production of such products, the polyamide materials are typically dyed a desired color. Polyamide materials have been conventionally dyed using acid dyes, which are anionic in character. Since acid dyes are negatively charged, the dyes are attracted to positive dye sites appearing in the targeted substrate. With respect to polyamides, positive dye sites can be created by exposing the free amino groups contained within the polymer matrix to an acid. In particular, when exposed to acidic conditions, the amino groups are activated by protonation and become positively charged and cationic. Once positively charged, the acid dyes are strongly attracted to the cationic sites.
In general, acid dyes have a high affinity for protonated polyamide materials meaning that the dyes have a strong tendency to quickly bind to the polymer. Unfortunately, however, once in contact with the cationic polymer surface, acid dyes have a tendency to poorly diffuse into the polyamide. In other words, acid dyes exhibit such a high rate of strike that they do not diffuse evenly into polyamides. Thus, if the dye is absorbed by the polymer too quickly, the polyamide material can absorb the dye unevenly and not exhibit a constant shade or color.
Consequently, polyamide materials are typically dyed with acid dyes under carefully controlled conditions in order to control the rate at which the dye is absorbed by the polymer. In particular, the temperature and the pH of the dye bath are usually monitored and regulated during the process. Specifically, increasing the temperature of the bath increases the diffusion rate, while controlling the pH controls the number of dye sites that are available for receiving the acid dye. For instance, at each pH of the dye bath, a distribution equilibrium exists between the polyamide material and the dye. At higher pH's, the dye is not readily accepted by the polymer. At lower pH's, on the other hand, equilibrium shifts and the dye becomes strongly attracted to the polymer.
In conventional acid donor systems for dyeing polyamides with acid dyes, the polyamide materials are placed in a dye bath initially containing an acid dye, a leveling agent, and an acid donor sufficient for dye exhaustion. Sometimes an alkaline composition is added in an amount sufficient to raise the pH of the bath to a level that inhibits initial absorption of the dye into the polymer. The dye bath is heated to promote the hydrolysis of the acid donor composition which decrease the pH gradually. Ideally, the pH of the bath is dropped at a rate which causes the dye to slowly diffuse into the polymer substrate. If the pH can be effectively controlled, the dye becomes evenly distributed throughout the bath and substrate and is absorbed by the substrate uniformly to create a polymer having a constant color and shade. Control of the dye bath pH is essential for the attainment of level and reproducible results.
In the past, various different agents have been used in order to control and gradually decrease the pH of dye baths utilized for dyeing polyamide materials. For example, an acetate buffer composed of acetic acid and either sodium or ammonium acetate has been used for pH control. Acetic acid, however, which is volatile, was found to vaporize during some dyeing processes. Substantial vaporization of the acetic acid caused the pH of the bath to drift upwards which resulted in uneven application of the dye.
Other agents that have been added to dye baths in the past for decreasing the pH of the bath during dyeing of polyamide materials include lactones as described in U.S. Pat. No. 3,980,428, an ester of a saturated C.sub.2 -C.sub.4 -carboxylic acid as disclosed in U.S. Pat. No. 4,252,531, and cyclic esters of sulfurous acid as disclosed in U.S. Pat. No. 4,813,971.
Although the above proposed compositions have shown some success in controlling the pH of dye baths, better controls are still needed. For instance, some pH regulators used in the past are not capable of lowering the pH of the dye bath to a level low enough to ensure complete exhaustion of the dyes used, which is especially important when darker shades are desired. Further, many pH control agents in the past have been expensive to produce and have not controlled the pH of the bath as well as could be desired. Consequently, a need currently exists for further improvements in compositions and processes designed to control the pH of dye baths during the application of dyes to polyamide materials, especially nylon 6 and nylon 66 fibers, textiles and carpets.